IP Multimedia Subsystem (IMS) is an IP multimedia service support subsystem put forward in release 5 of the 3rd Generation Partnership Project (3GPP) in order to provide multimedia services for mobile users through the Internet. The IMS is characterized by the following features: (1) an IP-based multimedia service and session control core network; (2) a public platform that supports various integrated services and is independent of any access technology and access mode; (3) use of a flexible Session Initiation Protocol (SIP) and standardized open interfaces, which makes it possible to support a wide range of services; (4) definition and perfection by multiple standardization organizations (the 3GPP/3GPP 2 defines the IMS network components and infrastructure, and accomplishes concordance as far as possible; the International Telegraph Union-Telecommunication Standardization Sector (ITU-T) and the European Telecommunications Standardization Institute (ETSI) adopt the IMS-based New Generation Network (NGN) or Next Generation Network (NGN) architecture, and make progress on that basis). It is generally believed in the industry that the IMS is a trend for the future.
Most users still use the traditional CS network currently, including PSTN, Integrated Services Digital Network (ISDN), and 2nd Generation Global System for Mobile Telecommunication (2G GSM). Therefore, it is important to implement interworking between the IMS network and the CS network, namely, implement the calls between the IMS user and the CS user. Important devices for interworking between the IMS network and the CS network are Media Gateway Control Function (MGCF), IMS Media Gateway (IMS-MGW), and Border Gateway Control Function (BGCF). The MGCF is responsible for interconnection of the control plane, and implements mapping between the SIP signaling and the Bearer Independent Call Control protocol (BICC) or ISDN User Part (ISUP). Accordingly, the IMS-MGW performs protocol conversion on user plane, terminates at the CS network, and provides conversion between the two types of termination. The MGCF uses the H.248 protocol to control the action of the IMS-MGW. The BGCF selects the location of the egress to the CS network when the IMS user initiates a call to a CS network user.
Currently, the CS network service platform is separated from the IMS network service platform, which makes it costly for the operator to operate the network and introduce new services. The IMS Centralized Service (ICS) project currently developed by the 3GPP attempts to let the IMS service platform support both the user CS access and the PS access. The essence of the ICS is: The CS network bears the real-time IMS session media (voice and video), and a transparent control channel is introduced between the User Equipment (UE) and the IMS network to support the IMS service control. The control channel may be implemented through Unstructured Supplementary Service Data (USSD) or SIP, where the USSD is a data communication mechanism of the CS network, and is generally applicable to communication between the UE and the server). An ICS server is introduced into the IMS network to support the ICS. The ICS server communicates with the Serving Call Session Control Function (S-CSCF) through an IP multimedia Subsystem Service Control (ISC) interface.
In the prior art, at the time of setting up a call between the UE in the CS network and the UE in the IMS network, if the UE in the CS network is holding a call, the UE needs to send a call hold request to the CS network to release the radio resources of the UE in the CS network. The call hold announcement has to be played by the CS network to the peer device, which does not meet the requirements in certain service scenarios, for example, the user who leaves the telephone set temporarily expects to send individualized music to the peer user, or the user at a sports event site expects to play the on-site commentary to the peer user without further bidirectional conversation.